(1) Field of the Invention
The present invention relates to a ferroelectric liquid crystal material. More specifically, it relates to a ferroelectric liquid crystal composition having quick response properties which comprises a smectic liquid crystal compound and an optically active compound, and an optical switching element which contains this composition.
(2) Description of the Prior Art
Liquid crystal compounds are widely used as materials for display elements, and most of these liquid crystal elements are on TN type display system and liquid crystal materials are in the state of nematic phase.
The TN type display system has advantages such as less tiredness of eyes and extremely small consumption of electric power because of non-emissive type, whereas it has disadvantages such as slow response and disappearance of display at Certain viewing angles. In recent years, this system is being improved in such a direction as to keep up characteristics of flat displays, and in particular, the quick response and the enlargement of viewing angle are demanded.
In order to meet these demands, improvements in liquid crystal materials have been attempted. However, as compared with other non-emissive type displays (e.g., electroluminescence displays and plasma displays), it is apparent that the TN type display system is much poorer in points of response time and the extent of viewing angle.
In order that characteristics of the liquid crystal display element, such as features of the non-emissive type and small consumption of electric power may be maintained and in order that quick response corresponding to that of the non-emissive type displays may be assured, it is essential to develop a novel liquid display system in place of the TM type display system. As one of such attempts, a display system in which the optical switching phenomenon of ferroelectric liquid crystals is utilized has been suggested by N. A. Clark and S. T. Lagerwall (see Appl. Phys. Lett.,36, P 899, 1980).
The presence of the ferroelectric liquid crystals has been announced for the first time in 1975 by R. B. Mayer et al. (see J. de Phys., 36, P 69, 1975), and from the viewpoint of structure, these liquid crystals belong to a chiral smectic C phase, a chiral smectic I phase, a chiral smectic F phase, a chiral smectic G phase and a chiral smectic H phase (hereinafter referred to simply as "S.sub.C * phase", "S.sub.I * phase", "S.sub.F * phase", "S.sub.G * phase" and "S.sub.H * phase", respectively). In the chiral smectic phase, molecules form a layer and incline to the surface of the layer, and a helical axis is vertical to this layer surface.
In the chiral smectic phase, spontaneous polarization takes place, and therefore, when DC electric field is applied to this layer in parallel therewith, the molecules turn round the helical axis in accordance with its polarity. The display element of the ferroelectric liquid crystals utilizes this switching phenomenon.
Nowadays, of the chiral smectic phases, much attention is particularly paid to the S.sub.C * phase. The display system in which switching phenomenon of the S.sub.C * phase is utilized can be further classified into two types: a birefringence type system using two polarizers and a guest/host type system using a dichoric dye.
Features of these display systems are:
(1) Response time is very short.
(2) Memory properties are present.
(3) Display performance is not so affected by viewing angle.
Thus, the display systems have the possibility of achieving a high-density display and are considered to be effectively utilizable in the display element. However, also in these display systems, there are now many problems to be solved.
For example,
(1) No compounds are not present which assume the S.sub.C * phase which can be stably operated at room temperature.
(2) The helical pitch is short.
(3) The achievement of alignment is very difficult.
(4) Spontaneous polarization is small, and response time is long.
Therefore, typical requirements for the ferroelectric liquid crystal material are as follows:
(1) The S.sub.C * phase should be stable in an extensive temperature range inclusive of room temperature.
(2) The helical pitch should be long.
(3) The spontaneous polarization should be large.
At present, any chiral smectic liquid crystal compounds in a single state which satisfy the requirements are not present, and thus one attempt is to provide a ferroelectric liquid crystal composition which satisfies the above-mentioned requirements comprising several chiral smectic liquid crystal compounds which can meet a part of the above-mentioned requirements, or non-liquid crystal compounds after mixing these several compounds and repeating by a trial and error method.
In addition to the ferroelectric liquid crystal composition comprising the ferroelectric liquid crystal compound alone, Japanese Patent Laid-Open Publication No. 195,187/1986 discloses a ferroelectric liquid crystal composition which can be prepared by mixing one or more compounds assuming a ferroelectric liquid crystal phase with fundamental materials of compounds and compositions assuming non-chiral smectic C, F, G, H and I phases (hereinafter referred to simply as "S.sub.C and other phases"). Furthermore, another ferroelectric liquid crystal composition is also reported in which one or more compounds having optical activity but not assuming any ferroelectric liquid crystal phase are mixed with fundamental materials of compounds and compositions assuming the S.sub.C and other phases (Mol. Cryst. Liq. Cryst., 89, p 327, 1982).
From these reports, it is definite that the ferro-electric liquid crystal composition can be prepared by mixing one or more compounds having optical activity with fundamental materials irrespective of whether or not the compounds to be mixed assume the ferroelectric liquid crystal phase.
The above-mentioned smectic liquid crystal mixture which comprises the fundamental material assuming at least one of the non-chiral S.sub.C and other phases and which assumes at least one of the S.sub.C and other phases will be hereinafter referred to as the base S.sub.m mixture.
The base S.sub.m mixture preferred in practice is a liquid crystal mixture assuming the S.sub.C phase in an extensive temperature range inclusive of room temperature. Components of the base S.sub.m mixture, are suitably selected from liquid crystal compounds such as phenylbenzole series, Schiff base series, phenylpyridine series and 5-alkyl-2-(4-alkoxypheny)-pyrimidine.
For example, in Japanese Patent Laid-Open Publication No. 291,679/1986 and the pamphlet of PCT International Publication W086/06401, the ferroelectric liquid crystal prepared by mixing 5-alkyl-2-(4-alkoxyphenyl)pyrimidine with an optically active compound can assume the S.sub.C * phase in a wide temperature range inclusive of room temperature, and in the former publication, it is also described that when an optical switching element contains the ferroelectric smectic liquid crystal material in which the above-mentioned pyrimidine derivative is used as the base S.sub.m mixture, the thus made switching element can have a shortened response time. Furthermore, in Japanese Patent Laid-Open Publication No. 291,679/1986, it is disclosed that the ferroelectric liquid crystal material comprising 5-alkyl-2-4'-alkylbiphenylyl)pyrimidine, 5-alkyl-2-(4-alkoxyphenyl)pyrimid ine and an optically active compound also assumes the S.sub.C * phase in an extensive temperature range including room temperature, and that it is effective for the improvement of the response time.
However, at present, the response times of the ferroelectric liquid crystal compositions in Japanese Patent Laid-open Publication No. 291,679/1986 and the pamphlet of PCT International Publication W086/06401 are still longer (300 to 500 .mu.sec) as compared with other type display elements such as the presently used light emission type displays, and thus the improvement of the response properties are further demanded.
Also as for helical pitch, some improvements have been made. The ferroelectric liquid crystal material having the long helical pitch can be obtained by mixing a chiral smectic liquid crystal compound in which a helical twist direction is right-handed with a chiral smectic liquid crystal compound in which the helix is left-handed, or mixing a compound having the S.sub.C phase with a chiral smectic liquid crystal compound (Japanese Patent Laid-open Publication Nos. 90,290/1985 and 195,187/1986).